Rankine cycles use a working fluid in a closed-cycle to gather heat from a heating source or a hot reservoir and to generate a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by transferring heat to a cold reservoir and pumped up to a heating pressure again to complete the cycle. Power generation systems such as gas turbines or reciprocating engines (primary system) produce hot exhaust gases that are either used in a subsequent power production process (by a secondary system) or are lost as waste heat to the ambient. For example, the exhaust of a large engine may be recovered in a waste heat recovery system used for production of additional power, thus improving the overall system efficiency. A common waste heat power generation system operating in a Rankine cycle is shown in FIG. 1.
The power generation system 1 includes a heat exchanger 2, also known as a boiler or evaporator, a turboexpander 4, a condenser 6 and a pump 8. In operation, beginning with the heat exchanger 2, an external heat source 10, e.g., hot flue gases, heats the heat exchanger 2. This causes the received pressurized liquid medium 12 to turn into a pressurized vapor 14, which flows to the turboexpander 4. The turboexpander 4 receives the pressurized vapor stream 14 and can generate power 16 as the pressurized vapor expands. The expanded lower pressure vapor stream 18 released by the turboexpander 4 enters the condenser 6, which condenses the expanded lower pressure vapor stream 18 into a lower pressure liquid stream 20. The lower pressure liquid stream 20 then enters the pump 8, which both generates the higher pressure liquid stream 12 and keeps the closed-loop system flowing. The higher pressure liquid stream 12 then flows in to the heat exchanger 2 to continue this process.
One working fluid that can be used in a Rankine cycle is an organic working fluid such as cyclopentane. An organic working fluid in this context is sometimes referred to as an organic Rankine cycle (ORC) fluid, and systems using ORC fluids are sometimes referred to as ORC systems. For safety reasons, it is undesirable to have cyclopentane interacting with ambient air.
Two types of leakage problems, for example, should be addressed in turboexpanders used in ORC systems. When the ORC systems are operating, the pressure inside the loop of FIG. 1 is relatively high as compared to the ambient environment, so it is desirable to contain the ORC fluid, e.g., cyclopentane from escaping (exfiltrating) the system. When the ORC systems are not operating, i.e., in standstill mode, then the pressure inside the loop of FIG. 1 is relatively low as compared to the ambient environment, so it is desirable to avoid the ambient air from infiltrating the ORC system.
Accordingly, systems and methods for more efficiently operating a power generation system are desirable.